Re: Simple circuit, stupid problem

On 21 Aug 2005 11:34:39 -0700, "Eriswerks" <eriswerks@xxxxxxxxx>
wrote:

>I'm trying to build a two transistor LED flasher, as shown at
>http://www.reprise.com/host/circuits/transistor_flasher.asp. It should
>be perfectly simple, but I must be making some mistake that I'm just
>not seeing yet.
>
>When I build this circuit, both LEDs light continuously rather than
>flashing. I've tried varying the values of the resistors and caps that
>should control the timing, but it doesn't make a difference. I can
>actually pluck the capacitors right out of the circuit and both LEDs
>are still lit. I've built this thing a dozen times now from several
>different people's instructions, and I get the same result every time.
>
>The transistors I have on hand are a little mixed... I've tried
>2N3904s, 3906s, and 4424, any of which ought to work as they're all
>NPN.
>
>Does anyone have any suggestions as to what stupid mistake I'm making
>here? I am starting to get sick of looking at this thing. Thanks in
>advance..!


This circuit has two stable states: "oscillating" is one, and "both
transistors saturated" is the other.

If the transistors have high beta, saturation is more likely. In this
example, if betas are above about 80, the thing can lock up, and lots
of transistors are hotter than that. Bigger base resistors might help,
but that has other problems.

If you return the tops of the 39Ks to their own transistor's
collectors, rather than to V+, that will prevent saturation lockup.

Or maybe something else is wrong. If you briefly short one base to
ground, then turn it loose, it should start oscillating. If not, look
elsewhere.

John

.

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